1. Field of the Invention
The present invention relates generally to a turbomachine, and more specifically to adjusting the flow capacity between airfoils in a turbomachine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A turbomachine, such as a compressor and a turbine, especially for those used in a gas turbine engine, include one or more stages of rotor blades in which each stage includes a stage of stator vanes or guide vanes located upstream of the rotor blades to guide the airflow into the rotor blades. In a gas turbine engine, the compressor is designed for a certain flow rate through the engine. The turbine is also designed to receive the flow rate from the compressor for maximum efficiency. The flow rate through a turbine is controlled by the throat area between adjacent stator vanes. In order to provide the highest efficiency for the engine, the flow rate through the turbine should be coordinated with the flow rate that the compressor and combustor would put out. If the turbine throat area is too large, the efficiency of the engine will drop. If the throat area is too small, pressure upstream of the turbine will increase and cause compressor surge which will also decrease the efficiency of the engine.
The prior art U.S. Pat. No. 6,109,869 issued to Maddaus et al on Aug. 29, 2000 and entitled STEAM TURBINE NOZZLE TRAILING EDGE MODIFICATION FOR IMPROVED STAGE PERFORMANCE discloses a process for altering the throat areas by cutting back selected portions of the trailing edge of the partitions in order to minimize or eliminate interaction of the tip and hub vortices in the hot steam flow path or to reduce additional secondary aerodynamic flow losses (see column 2, line 46 of this patent). The Maddaus patent addresses changing the radial distribution of the airfoil throat area (as shown in FIG. 6 of this patent) in order to increase stage performance.
Another prior art process, that of U.S. Pat. No. 4,741,667 issued to Price et al on May 3, 1988 and entitled STATOR VANE, discloses varying the stator vane throat area in order to achieve a radial distribution of throat area while keeping a straight airfoil leading/forward edge section for the purpose of using inserts within the airfoil. According to the Price et al patent, “a stator vane configuration is provided with a chordal dimension varying over the span of the vane from a maximum value proximate the vane midspan and decreasing radially inwardly and outwardly therefrom. When arranged in a stage with a circumferentially distributed plurality of similarly configured vanes, the vane configuration according to the present invention achieves a radially varying nozzle throat size for inducing a greater working fluid mass flow adjacent the radially inner and outer vane ends. The flow modification thus induced results in a more desirable working fluid axial velocity profile entering the downstream rotor stage.” See column 2, lines 47-59 in this patent.
In a medium to large gas turbine engine, the turbine stator vanes are cast with such a tolerance that the throat area is generally within the range to provide the proper flow capacity for high efficiency of the engine. In a typical gas turbine engine of this class, airfoil tolerance requirements are set such that the resulting effective throat areas are within about 2% to 3% of the intended design. However, in a small gas turbine engine, because the airfoils (blades and vanes) are so thin, the tolerances of these small airfoils could result in throat areas that far exceed the flow design levels and result in poor engine performance.
It is therefore an object of the present invention to provide for a small gas turbine engine that has a flow capacity close to the design parameters for a high efficiency engine.
It is another object of the present invention to shape an airfoil such a smooth relation exists between removal of trailing edge material and effective flow area.